The present invention relates to a flame-retardant intumescent coating for wood-based building products. More particularly, the invention provides a fast-curing flame-retardant coating that expands when exposed to fire to form a protective insulating layer over a wood-based substrate, and a wood-based product coated with the coating.
Flame-retardant treatments have been used on wood-based building products for many years to enhance the safety of structures built with the products. Several different types of flame-retardant treatments are known. For example, one type of treatment involves impregnating the building products with flame-retardant chemicals. Another type involves coating the surface of the building products with a flame-retardant coating, and another type involves bonding flame-retardant laminates to the surfaces of the building products. Each of these treatments has several different flame-retardant functions. First, the treatments may exclude oxygen from the underlying wood, reducing the tendency of the wood to burn. Thus, a structure built with treated materials may suffer less damage in a fire than a structure built with untreated materials. Second, the treatments may reduce the amount of smoke produced in a fire, and possibly lower the danger of smoke inhalation by persons in a burning structure. Third, the treatments may reduce the overall thermal flux of a fire, and thus may reduce the tendency of a fire to spread.
Early flame-retardant treatments often involved impregnating the building product with a flame-retardant chemical, such as monoammonium phosphate or phosphoric acid. These chemicals react with the wood in the presence of heat so that the wood forms a char layer. The char provides a protective layer that insulates the underlying wood and prevents oxygen from reaching the underlying wood. However, these treatments have several drawbacks. First, the formation of the char weakens the wood. Second, the phosphates may leach out of the wood with time, reducing the effectiveness of the treatments. Third, these treatments have been known to cause premature thermal degradation of the wood in the presence of heat from a source such as solar radiation, which may possibly lead to the failure of a structure made from the treated wood.
To avoid the loss of strength associated with the char formation in chemically impregnated flame-retardant building products, some building products are coated with a flame-retardant intumescent, or expandable, coating. In these systems, the coating itself forms a char, so that char formation does not damage the strength of the building product. These coatings generally consist of an inorganic acid such as phosphoric acid, an organic material that may be dehydrated by the acid to form a char, a blowing agent to evolve a gas in the presence of heat to expand the char, and a binder to hold the char together and to bind the char to the underlying wood-based material. The expanded coating insulates the underlying material and prevents oxygen from reaching the underlying wood. These coatings are an improvement over the wood-impregnating treatments because the char is not formed from the wood itself. However, these coatings also have some drawbacks. First, the phosphates in the coatings are expensive, and significantly increase the cost of building materials treated with the coatings. Second, the char layer may be too thin to insulate the underlying wood well enough to prevent damage. Third, many of these coatings have insufficient expansive strength to expand when covered by paint or other wood treatments. Fourth, many of these coatings must be applied as aqueous emulsions. These emulsions may not be suitable for production line use, as the dry times of the coatings may be quite slow.
Another type of intumescent coating uses a graphite-based substance known as expandable graphite to protect the underlying wood-based material from fire. Expandable graphite is produced by intercalating a material that decomposes into a gas when heated into the crystal lattice of ordinary graphite. When the coating is heated to the decomposition temperature of the intercalate, the gasses produced push apart the layers of the graphite, expanding the coating up to 100 times the original thickness and producing an insulating layer over the wood-based material.
Coatings with expandable graphite typically expand to a greater degree than other intumescent coatings, and thus provide superior insulation to the underlying material. However, known expandable graphite coatings also have drawbacks. First, these coatings often exhibit the "popcorn effect", where the expansion of the grains of the graphite cause the grains to pop off the surface of the underlying material. Second, the expanded graphite is weak and brittle, and may be blown off of the underlying material by the turbulent winds produced in a fire. A binder is typically used to prevent the popcorn effect and to hold the expanded coating together. However, many binders are flammable, and thus may counteract the flame-retardant properties of the coating.
One example of a flame-retardant coating with expandable graphite is disclosed in PCT Publication No. WO 91/11498. The coating comprises expandable graphite, one or more binders and one or more of a range of intumescent char-forming materials, defined in the publication as a mixture including phosphates or sulfates and a carbonific material, such as a starch or a polyhydric alcohol. However, the phosphates and sulfates used in these coatings are relatively expensive, and may increase the price of building materials treated with these coatings. Another example of an expandable graphite coating is disclosed in U.S. Pat. No. 5,968,669. The coating comprises expandable graphite, miscellaneous fillers, a polymeric binder and a wetting agent. Because this coating is applied as a water-based solution, the dry times of the coating may be too slow for production line use. Thus, there remains a need for an inexpensive, strongly adhering intumescent coating for wood-based building products that may be applied at a high process rate.